Which one of the following sensory systems uses unmyelinated fibers to convey information to the central nervous system?

The thermoreceptors are on the free nerve endings of unmyelinated C fibers and small myelinated (Ad) sensory fibers. Proprioception (i.e., the sense of muscle tension and length) is conveyed by la and lb afferent fibers, which innervate muscle spindles and Golgi tendon organs. Vibration is detected by Pacinian corpuscles and pressure is detected by Ruffini's corpuscles and pressure is detected by Ruffini's corpuscles. Both are innervated by large, myelinated (group II) afferent neurons.